1. Field of the Invention
This invention relates to the field of making mixed metal hydroxides or layered double hydroxide products. More specifically, the invention relates to an improved two-step method for making hydrotalcite and hydrotalcite-like compounds from dry powder constituents.
2. Technology Review
Hydrotalcite exists in both a natural and synthetic form. Naturally occurring deposits have been found in Snarum, Norway and in the Ural Mountains. Typical occurrences are in the form of serpentines, talc schists, or as an alteration product where hydrotalcite forms the pseudomorph of a spinel. Like most ores, natural hydrotalcite is virtually impossible to find in a pure state. Such deposits often contain one or more other minerals including penninite and muscovite.
Several methods are known for making synthetic hydrotalcite in such product forms as a fine powder, -20 mesh granules or as 1/8-inch diameter extrudates. One representative method is described in U.S. Pat. No. 3,539,306. There, an aluminum hydroxide, aluminum-amino acid salt, aluminum alcoholate, water soluble aluminate, aluminum nitrate and/or aluminum sulfate are mixed with a magnesium component selected from magnesium oxide, magnesium hydroxide or water-soluble magnesium salt and a carbonate ion-containing compound in an aqueous medium maintained at a pH of 8 or more. The resulting product may be used as a stomach antacid. In this typical neutralization process, a fairly pure, finely sized hydrotalcite particle is formed. A serious disadvantage of this method, however, is its formation of a sodium salt by-product. This salt neutralization process for making hydrotalcites could also produce a brucite-like structure with undesired anions (e.g. sulfate) or cations (Na.sup.+) included therein.
In Misra Reissue U.S. Pat. No. 34,164, the disclosure of which is fully incorporated by reference, yet another means for synthesizing hydrotalcite is taught. The method comprises heating magnesium carbonate and/or magnesium hydroxide to form activated magnesia, then combining the activated magnesia with an aqueous solution of aluminate, carbonate and hydroxyl ions.
Other known methods for synthesizing hydrotalcite include: adding dry ice or ammonium carbonate to a thermal decomposition product from a magnesium nitratealuminum nitrate mixture, after which intermediate product is subjected to temperatures below about 325.degree. F. and pressures of 2,000 to 20,000 psi. Yet another process, from "Properties of a Synthetic Magnesium-Aluminum Carbonate Hydroxide and its Relationship to Magnesium-Aluminum Double Hydroxide Manasseite, and Hydrotalcite", The American Mineralogist, Vol. 52, pp. 1036-1047 (1967), produces hydrotalcite-like materials by titrating a solution of MgCl.sub.2 and AlCl.sub.2 with NaOH in a carbon dioxide-free system. This suspension is dialyzed for 30 days at 60.degree. C. to form a hydrated Mg-Al carbonate hydroxide having the properties of both manasseite and hydrotalcite.
It is a principal objective of this invention to provide an improved means for making synthetic hydrotalcite and hydrotalcite-like compounds from two or more relatively inexpensive, dry powder components. It is another objective to provide an improved process for making hydrotalcite and related materials with less sodium ion contamination. It is still another objective to provide a method for synthesizing hydrotalcite without depending on the use of any alumina gels. It is still another objective to make hydrotalcite and hydrotalcite-like compounds through the further processing of an improved meixnerite product, itself made by combining activated magnesia with a high surface area, transition alumina.
Yet another principal objective is to make hydrotalcite and hydrotalcite-like compounds in a more environmentally acceptable manner. According to preferred embodiments, the synthetic hydrotalcites made by the methods described hereinbelow yield no by-products other than water. Any remaining discharge waters should be easily disposable due to their low dissolved solids content.